In general, RFID is a system that allows for automatic identification of RFID tags and things attached thereto. An RFID system usually includes a number of RFID tags that may be used to identify and/or track objects, persons, animals, products, inventory, and so forth, and at least one RFID reader that may be used to detect and read RFID tags. Typically, there may be two types of RFID tags: active RFID tags that contain a power source (a battery, for example) and passive RFID tags that do not contain a power source, but derives its power from received transmissions of an RFID reader.
Since passive RFID tags do not contain a power source and derive their power from received transmissions, the passive RFID tags will typically not be able to derive any power to operate when transmissions are not received. Therefore, volatile information, such as data stored in dynamic memory memories, including dynamic random access memory, registers, latches, so on, for example, will be lost after transmissions are no longer received at the passive RFID tags, resulting in a loss of power.
FIG. 1 illustrates a diagram of an inventory system 100 utilizing RFID tags and RFID readers, wherein loss of volatile information may impact the performance of inventory system 100. Inventory system 100 includes a number of RFID readers 105-108 that may be used to identify and track inventory. The inventory may include individual product units or pallets of products, with each unit or pallet containing one or more passive RFID tags. A passive RFID tag included with a unit or pallet may contain information pertaining to the unit or pallet, such as identification number, content, count, and so forth. As shown in FIG. 1, only passive RFID tags, such as RFID tag “TAG3” 115, “TAG7” 116, “TAG11” 117, and “TAG13” 118, are shown. The actual product or pallets containing the passive RFID tags are not shown to maintain simplicity.
A first time an RFID tag, such as TAG3 115, passes within range of an RFID reader, such as RFID reader 105, transmissions by RFID reader 105 may energize TAG3 115. This may enable TAG3 115 to respond to inquiries made by RFID reader 105. Part of the communication process between RFID reader 105 and TAG3 115 may include a handshake operation that may include identification of TAG3 115, entry of TAG3 115 into inventory system 100, marking TAG3 115 as an RFID tag that has been identified by inventory system 100, and so on. The handshake operation may take a finite amount of time.
After TAG3 115 completes its handshake operation with RFID reader 105, TAG3 115 may provide information about product content, product identification information, product count, intended destination, and so forth, to RFID reader 105 depending on inquiries from RFID reader 105. Based on the information retrieved from TAG3 115, inventory system 100 may direct the movement of TAG3 115 and product units or pallet of product to which it is attached.
In order to help with the performance of inventory system 100, a record of TAG3 115 already establishing communications with inventory system 100 may be stored in both inventory system 100 as well as TAG3 115. This may prevent TAG3 115 from having to repeatedly perform the handshake operation as well as responding to information requests from RFID readers in inventory system 100 as it moves along in inventory system 100. However, since TAG3 115 is a passive RFID tag, once TAG3 115 moves outside of communications range of RFID reader 105, it may no longer be able to derive power needed to maintain volatile information stored therein. Therefore, it may be possible that by the time that TAG3 115 passes within range of RFID reader 107, for example, the volatile information may have disappeared, and TAG3 115 may need to repeat the handshake operation as well as respond to information requests. This may impact performance of inventory system 100. Therefore, there is a need to maintain the volatile information stored in a passive RFID tag for a period of time after power is no longer available to the passive RFID tag. For example, in order to help prevent loss of volatile information due to unexpected power shielding or power interruption during a handshaking operation with an RFID reader, RFID technical specifications specify that a passive RFID tag should be able to maintain the volatile information for about two (2) seconds.
FIG. 2a illustrates a prior art technique for maintaining volatile information in a passive RFID tag 200. RFID tag 200 includes a capacitor 205 that is used to store the volatile information (a single bit as shown in FIG. 2a). The volatile information may be written to capacitor 205 by a combination of charge circuit 210 and supplemental discharge circuit 215. A comparator 220 may compare a charge stored on capacitor 205 with electrical ground to determine a value of the volatile information stored in passive RFID tag 200. A leakage circuit 225 may be used to maintain the volatile information when passive RFID tag 200 is not powered.
FIG. 2b illustrates a circuit 250 that is an embodiment of the prior art technique for maintaining volatile information shown in FIG. 2a. Circuit 250 includes a plurality of CMOS inverters 255-257 to control NMOS gates 260 and 261. NMOS gate 261 may be an implementation of charge circuit 210 and CMOS inverter 256 may be used to keep leakage current low during power up. VREF powers CMOS inverter 256, while VDD powers CMOS inverter 257. When VDD is sufficiently higher than VREF, a capacitor used to store the volatile information is charged to VREF. A discharge path through NMMOS gate 260, different from the charge path, may allow for a discharge of the capacitor. The volatile information stored in the capacitor may be maintained by opening NMOS gates 260 and 261 when power is lost.